1. Field of the Invention
This invention relates to a brushless DC motor having a magnet magnetized so as to have a plurality of magnet poles in the circumferential direction thereof for use in a small funxe2x80xa2blower and, more particularly, relates to a brushless DC motor comprising two stator yokes, each having main magnetic pole and magnetic pole pieces, a center yoke passing through the center portions of the two stator yokes for connecting magnetically the two stator yokes, and an annular winding arranged between the two stator yokes.
2. Description of the Prior Art
Japanese Utility Model Laid-Open No. 153486/86, Japanese Patent Laid-Open No. 63065/83, Japanese Patent Laid-Open No. 214759/86, Japanese Patent Laid-Open No. 23754/89, and Japanese Patent Laid-Open No. 303750/94 propose many kinds of the brushless DC motor.
FIG. 8 is an exploded view of the conventional brushless DC motor disclosed in the Japanese Patent Laid-Open No. 23754/89. FIG. 9 shows a relation between the magnet and the stator yokes. FIG. 10 is a sectional view of the conventional brushless DC motor.
The brushless DC motor of four poles shown in FIG. 8 has a rotor 1 consisting of a cylindrical rotor yoke 2 of the magnetic material, a magnet 3 magnetized so as to have a plurality of magnet poles in the circumferential direction thereof, and a motor shaft 4. Reference numeral 5 denotes a first stator yoke having two main magnetic poles 13 and magnetic pole pieces 21. Reference numeral 23 denotes a bobbin, 24 denotes an annular winding wound around the bobbin 23. Reference numeral 25 denotes terminals mounted on the bobbin 23, to each of the terminals 25 each of lead wires of the winding 24 being tied up and soldered, 6 denotes a second stator yoke having two main magnetic poles 14 and magnetic pole pieces 22. Reference numeral 7 denotes cylindrical center yokes formed on center portions of the first and second stator yokes 5 and 6, inserted telescopically each other so as to connect magnetically the first and second stator yokes 5 and 6 each other, 17 denotes a slit formed axially on each of the center yokes 7, 9 denotes an electromagnetic conversion element, such as a Hall element for detecting the magnet pole of the magnet 3, 11 denotes a printed wiring board for connecting electrically the Hall element 9 and the terminals 25 to a driving circuit.
FIG. 11 shows a driving circuit for two-phase half-wave electric currents energizing alternately two windings 24a and 24b having a phase difference of 180xc2x0 in electrical angle. In FIG. 11, reference numeral 60 denotes a motor portion, 61 and 62 denote transistors each driven by a signal from a motor driving IC 63, 64 denotes a DC power source, and 65 denotes electric parts or IC parts installed on the printing wiring board 11.
In the Japanese Patent Laid-Open No. 23754/89, on page 4, lines 30-38 and in FIG. 3 thereof, a figure and an arrangement of magnetic pole pieces of first and second stator yokes are disclosed, as shown in FIG. 9 of the present application.
The motor shown in FIG. 9, however, has a dead point at which the motor cannot be driven in principle due to the specific arrangement of the magnet and the magnetic pole pieces. In order to avoid such dead point and to reduce a torque ripple during the rotation of the motor, a relative position of the magnet and the magnetic pole pieces is considered.
Specifically, it is stated in the prior art that it is preferable to reduce a spread angle a of the magnetic pole piece to ⅕ to ⅘ of that of one magnet pole of the magnet 3, and to set a spread angle b formed between the center lines of the magnetic pole pieces 21 and 22 of the first and second stator yokes 5 and 6 to xcfx80/4 to 3xcfx80/4 in electrical angle.
Further, it is suggested in the publication that it is preferable for a high speed brushless DC motor to provide a slit 17 in the axial direction in each of the center yokes 7, in order to reduce the eddy current and the eddy current loss produced by the magnetic flux in the axial direction.
In the Japanese Patent Laid-Open No. 303750/94, on page 3, line 34 to page 4, line 12, it is pointed out that the sum of the current torque and the cogging torque becomes the motor torque, and there is a problem in the negative torque and the ripple of the motor torque, and proposed that the spread angles a and c of the magnetic pole pieces of the first and second stator yokes shown in FIG. 9 of the present application are different from each other. Further, in the Japanese Patent Laid-Open No. 63065/83, it is suggested that a projecting portion 20 is provided on each of magnetic pole pieces of the first and second stator yokes so as to project in a direction reverse to the rotary direction of the magnet different from the above prior art, as shown in FIG. 12 of the present application.
However, in the conventional motor as mentioned above has following defects.
In the conventional brushless DC motor disclosed in the Japanese Patent Laid-Open No. 23754/89, the winding 24 is wound around the bobbin 23, the lead wires of the winding 24 are tied up and soldered to the terminals 25 mounted on the printed wiring board 11, as shown in FIG. 8 of the present application. In such structure, the cost for the parts, such as the bobbin 23 and the terminals 25 is increased. Further, the treatment of the lead wires of the winding and the soldering thereof are complicated and the cost thereof is increased.
Further, in the above brushless DC motor, it is necessary to use the IC driving circuit for driving the two-phase bifilar windings 24a and 24b having a phase difference of 180xc2x0 in electrical angle. The space factor of the winding 24 is reduced remarkably because two wires are jumbled together when the bifilar winding is wound around the bobbin.
Further, the magnetic flux from the magnet cannot be used effectively, because the space angles of the first and second stator yokes are different from each other and thus the spread angle a of the magnetic pole piece becomes small with respect to that of the one magnet pole of the magnet. Further, in the arrangement shown in FIG. 9, a magnetic flux from the N pole of the magnet 3 is returned to the S pole of the magnet 3 through the first stator yoke 5, the center yokes 7 and the second stator yoke 6. However, the magnetic flux from the N pole to the S pole of the magnet 3 is limited and entire magnetic flux cannot be returned to the S pole of the magnet, because a portion of the magnetic pole piece of the second stator yoke 6 is positioned at the boundary of the N and S poles of the magnet. Further, the effective interlinkage magnetic flux and thus the motor efficiency are reduced, because the magnetic flux passing through the center yokes 7 is interlinkaged with the annular winding.
FIG. 7 is a graph showing relations between the interlinkage magnetic flux of the motor winding and the counter electromotive force with respect to the motor revolution, in case that the materials and figure of the center yoke are varied. In FIG. 7, a curve {circle around (1)} shows the counter electromotive force in case that an electromagnetic soft iron is used, and {circle around (4)} shows the interlinkage magnetic flux. The frequency of the alternating magnetic flux passing through the center yoke becomes high and the interlinkage magnetic flux is suppressed by the eddy current according to the increase of the rotation, so that the counter electromotive force is saturated. In FIG. 7, a curve {circle around (2)} shows the counter electromotive force and {circle around (5)} shows the interlinkage magnetic flux in case that one slit 17 is formed axially in each of the center yokes 7 as shown in FIG. 8. By the effect of the reduction of the eddy current, the counter electromotive force and the interlinkage magnetic flux are improved in the high speed rotation region, however, the interlinkage magnetic flux is reduced in the low speed rotation range. Further, if the slit is formed in the center yoke, the precision of the outer dimension of the center yoke is lowered.
In the Japanese Patent Laid-Open No. 303750/94, the spread angle c of the magnetic pole piece 22 of the second stator yoke 6 is smaller than the spread angle a of the magnetic pole piece 21 of the first stator yoke 5, as shown in FIG. 9 of the present application, so that the width of the magnetic pole piece 22 becomes small, the effective interlinkage magnetic flux of the motor is reduced, and the motor efficiency is also reduced.
In the Japanese Patent Laid-Open No. 63050/83, the area of the magnetic pole piece is reduced and the effective interlinkage magnetic flux is also reduced, because the projecting portions 20 is provided on the magnetic pole pieces 21 and 22, as shown in FIG. 12 of the present application. Further, in such DC motor, there is such a fear that the magnetic flux passing through the electromagnetic steel plate of each of stator yokes is saturated, because the thickness of the electromagnetic steel plate is limited in most cases. The magnetic flux from the magnet 3 is passed through the tip end of the magnetic pole piece to the main magnet pole, and the density of the magnetic flux is large, so that there is a possibility of saturation of the magnetic flux if the passage of the magnetic flux is narrowed.
When an electric current is passed through the annular winding, not only the rotary force is generated between the projecting portions 20 of the magnetic pole pieces 21 and 22 and the magnet 3, but also the attractive force and the repulsive force directed to the stator yoke are generated, so that the magnetic pole pieces are oscillated. If the width of the connecting portion of the main magnetic pole 13 and the magnetic pole piece 21, as well as the width of the connecting portion of the main magnetic pole 14 and the magnetic pole piece 22 are small, the oscillation becomes large and thus the noise is increased.
It is preferable that the wind passage area is increased and the motor revolution number is reduced as much as possible in order to maintain the large quantity of wind, to increase the efficiency and to reduce the noise in the small fanxe2x80xa2blower. FIG. 13 is a perspective view of a small fan motor. A reference numeral 50 denotes a fan case and 51 denotes blades provided on the rotor yoke 2. In FIG. 13, it is preferable that the outer diameter of the rotor yoke 2 of the brushless DC motor positioned at the center of the fan is reduced to a value lower than 55% of the outer diameter of the blade 51, in order to increase the quantity of the suction wind.
However, in the small motor having the annular winding as shown in FIG. 10, the thickness of the bobbin 23 used for winding of the winding 24 and the insulation thereof is larger than about 0.5 mm. Accordingly, the space of the winding 24 is reduced by the thickness of the bobbin 23. For example, in case of a motor wherein the outer diameter A is about 18 mm and the length D of the stator yoke is about 15 mm, the space factor of the winding becomes lower than 80%, and the output and the efficiency of the motor are reduced. Further, the diameter of the motor cannot be reduced, because the space for the bobbin 23 is necessary. As a result, the blade of the fanxe2x80xa2blower becomes small and the efficiency is lowered. Further, the noise due to the motor oscillation is increased if the motor input is increased in order to increase the quantity of wind of the fanxe2x80xa2blower.
The winding 24 itself is heated by the electric current passing through the winding 24 when the motor is driven. In general, the bobbin 23 is formed of the resin and thus the heat conductivity is small, so that the heat radiation from the stator yokes 5 and 6 is small, the temperature of the motor is increased, and the life time of the bearing of the fanxe2x80xa2blower is shortened.
An object of the present invention is to solve the above technical problems.
Another object of the present invention is to provide a small motor of a high efficiency suitable for use in the small fanxe2x80xa2blower, wherein the number of the parts is reduced, the cost is reduced remarkably by improving the treatment of the lead wires of the winding, the reduction of the effective interlinkage magnetic flux of the motor is prevented, and the oscillation of the magnetic pole piece is prevented.
A further object of the present invention is to provide a brushless DC motor, wherein a winding assembly is composed of an annular winding, an electromagnetic conversion element, and a driving circuit mounted on a printed wiring board; main magnetic poles of a first stator yoke is deviated by about 180xc2x0 in electrical angle from main magnetic poles of a second stator yoke; both end surfaces of the winding assembly are held by the main magnetic poles of the first and second stator yokes; an outer peripheral surface of the annular winding is covered with magnetic pole pieces of the first and second stator yokes; and a center yoke is positioned at the center of the winding assembly.
The annular winding is an air-core winding and formed of two normally tight wound self welding wires, each wire having an adhesive film around an insulating coating thereof, the winding being energized by two-phase half-wave electric currents having a phase difference of 180xc2x0 in electrical angle.
The printed wiring board is flexible.
At least one of the first and second stator yokes is movable in the axial direction thereof so as to meet with a change of the length of the annular winding.
At least one of the magnetic pole pieces of the first and second stator yokes has a cutout portion so as to reduce gradually an area of the magnetic pole piece toward the tip end thereof.
The center yoke is made of a soft magnetic stainless steel comprising 10-15 wt % of chrome and 85-90 wt % of iron.
Still further object of the present invention is to provide a brushlees DC motor, wherein two sets of stator yokes are coated with thin insulating films, and an annular winding formed of self welding wires is inserted into a space formed between the center yoke and the magnetic pole pieces without forming any gap substantially therebetween, so that the space factor of the winding becomes near 100%.
The stator yokes and the annular winding are impregnated with a liquid insulating material and connected each other so as to increase the heat radiation of the annular winding.
A thin insulating sheet instead of the thin insulating coating for the first and second stator yokes is used.
Yet further object of the present invention is to provide a brushless DC motor for use in a driving source of a small fanxe2x80xa2blower in order to increase the efficiency remarkably and reduce the noise.
These and other aspects and objects of the present invention will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following description, while indicating preferred embodiments of the present invention, is given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.